Thermal, Structural, and Phase Evolution of the Y2(SO4)3*8H2O–Eu2(SO4)3*8H2O System via Dehydration and Volatilization to Y2(SO4)3–Eu2(SO4)3 and Y2O2(SO4)–Eu2O2(SO4) and Its Thermal Expansion

The synthesis, crystal structure, phase transformations, and thermal expansion of (Y1−xEux)2(SO4)3*8H2O (where x = 0, 0.17, 0.33, 0.50, 0.66, 0.83, and 1) are presented. (Y1−xEux)2(SO4)3*8H2O solid solutions were synthesized via crystallization from an aqueous solution. (Y1−xEux)2(SO4)3*8H2O (C2/c) ↔ (Y1−xEux)2(SO4)3 (Pbcn) → (Y1−xEux)2O2SO4 (C2/c) and Eu2(SO4)3*8H2O (C2/c) ↔ Eu2(SO4)3 (C2/c) → Eu2O2SO4 (C2/c) phase transformations for all samples were investigated by high-temperature powder X-ray diffraction, differential scanning calorimetry and thermogravimetry in the temperature ranges of 25–750 and 25–1350 °C, respectively. The aim of this work is to identify the structural heredity of the phases formed during thermal transformations of (Y1−xEux)2(SO4)3*8H2O solid solutions, and to study the mechanisms of the thermal deformations of the crystal structure. Structural relations between these phases were found. The crystal structures of YEu(SO4)3*8H2O and (Y0.83Eu0.17)2(SO4)3*8H2O were refined at −173, −123, −73, −23, 27, and 77 °C. Thermal expansion coefficients for (Y1−xEux)2(SO4)3*8H2O, Eu2(SO4)3, (Y1−xEux)2O2SO4 (where x = 0, 0.17, 0.33, 0.50, 0.66, 0.83, and 1) compounds and solid solutions were calculated for the first time. The thermal expansion of Eu2(SO4)3 was highest in the direction approximately coinciding with the c-axis, because the Eu–O chains extended in this direction. As temperature increased, the crystal structure of (Y1−xEux)2(SO4)3*8H2O expanded significantly in the ac plane along directions close to the a and c axes, while thermal expansion along the b axis was relatively low. The distance between layers in the (Y1−xEux)2(SO4)3*8H2O crystal structure increased with increasing temperature, and corrugated layers (parallel to (101) direction) straightened out due to the rotation of the S2O4 tetrahedra. At high temperature, thermal expansion of Y2O2SO4 was highest along the longer diagonal of the ac parallelogram perpendicular to the plane of the oxo-centered 2∞[YO] layers.